1. Field of the Invention
The present invention is generally directed to display devices and, more particularly, to active matrix liquid crystal display devices in which pixels (e.g., picture elements or picture cells) are formed by use of thin film transistors and pixel electrodes.
2. Description of the Invention
An active matrix liquid crystal display device includes a liquid crystal display unit on which a plurality of pixels are arranged in matrix form. Each individual pixel on the liquid crystal display unit is disposed in each of intersection regions defined by two adjacent scanning signal lines (gate signal lines) and two adjacent image signal lines (drain signal lines). The plurality of scanning signal lines extending in the row-direction (horizontal direction) are arrayed in the column-direction, while the plurality of image signal lines extending in the column-direction (vertical direction), intersecting the scanning signal lines, are arrayed in the row-direction.
The pixel is formed mainly of a liquid crystal in combination with a thin film transistor (TFT), a common transparent pixel electrode and a transparent pixel electrode which are disposed through the liquid crystal. The transparent pixel electrode and the thin film transistor are each provided for every pixel. The transparent pixel electrode is connected to a source electrode of the thin film transistor. A drain electrode of the thin film transistor is connected to the image signal lines, while a gate electrode is connected to the scanning signal lines.
A typical arrangement is such that unnecessary incident light emerging from a panel front surface is shielded by a light shielding film formed on the upper portion of TFT, and beams of backlight which are not required are shielded by the non-transparent gate electrode. In accordance with a variety of experiments performed, the present inventors have found that sufficient light shielding effects cannot be obtained by a TFT gate electrode of an ordinary size.
When the light strikes upon an amorphous semiconductor layer of the thin film transistor, electron-hole couplings are generated, thereby deteriorating OFF-characteristics of the transistor. Hence, it is required that the amorphous semiconductor layer be arranged so as not to undergo the irradiation of light as much as possible. The light for display is classified into two types: natural incident light (or light of a room lamp) emerging from the front surface of the liquid crystal display panel and incident backlight of a fluorescent lamp which emerges from the underside of the panel.
The above-described liquid crystal display device tends to increase in the size of a pixel thereof, as the liquid crystal display unit is correspondingly increased in configuration. For instance, the size of pixel of the conventional liquid crystal display unit was 0.2×0.2 (mm2). However, the present inventors have developed a liquid crystal display device having a pixel size of 0.32×0.32 (mm2).
In this type of liquid crystal display device, foreign substances such as dust or the like are intermixed in the liquid crystal display device in the manufacturing process, or the foreign substances are adhered to a mask for use with photolithography. If the foreign substances are present or intermixed in between the source electrode (or transparent pixel electrode) and the drain electrode of the thin film transistor, short-circuiting takes place between these electrodes, resulting in a so-called point defect in which the short-circuited pixel is deteriorated. If the foreign substances are likewise present or intermixed in between the source electrode (transparent pixel electrode) and the gate electrode of the thin film transistor, the same point defect is caused. From this phenomenon, the present inventors have found out such a problem that the point defect (e.g., a loss of pixel), inherent in the above-described liquid crystal display device, becomes conspicuous, as each individual pixel increases in size.
Incidentally, the arrangement that a configuration of the gate electrode is made larger than the semiconductor layer has already been known in Japanese Patent Laid-Open Publication No. 17962/1985. However, even when simply increasing the size of the gate electrode, a parasitic capacitance between the gate electrode and the source electrode also increases, and a DC component applied to the liquid crystal due to scanning signals is increased. In all, the undesirable results become so prevalent that utilization is difficult.
An example of an active matrix liquid crystal display device is described on, e.g., pp. 193 to 200 of NIKKEI ELECTRONICS issued on Dec. 15, 1986, published by Nikkei McGraw-Hill Co., Ltd.
The following listings are exemplary of the pixel dividing technique in the active matrix liquid crystal display device: Japanese Patent Laid-Open Publication Nos. 49994/1982, 78388/1984, 97322/1985 and 77886/1986.